I feel like traveling is very hard on my body. In particular, my stomach seems to suffer a great deal. I feel like it is from pressure changes, but maybe it is from stress. How to know & reduce impact?

It could be but note. Note that you are taking Aspirin and advil. These are potent medicines and often cause some degree of gastritis which will cause a burning uncomfortable sensation in your upper abdomen. Stress is definitely harmful. So, slow down, do some relaxation type of exercises like daily walks. Simplify your life and use antacids like zantac (ranitidine) when your stomach is uncomfortable.